Flexography is a method of printing that is commonly used for high-volume relief printing runs on a variety of substrates such as paper, paperstock board, corrugated board, polymeric films, labels, foils, fabrics, and laminates. Flexographic printing has found particular application in packaging, where it has displaced rotogravure and offset lithography printing techniques in many cases.
Flexographic printing members are sometimes known as “relief printing members” and are provided with raised relief images onto which ink is applied for application to a receiver element of some type. The raised relief images are inked in contrast to the relief “floor” that remains free of ink. Such flexographic printing members (such as flexographic printing plates) are supplied to the user as an article having one or more layers optionally on a substrate or backing material. Flexographic printing can be carried out using flexographic printing plates as well as flexographic printing cylinders or seamless sleeves having a desired relief image.
Generally, flexographic printing members are produced from a photosensitive resin or elastomeric rubber. A photo-mask, bearing an image pattern can be placed over the photosensitive resin sheet and the resulting masked resin is exposed to light, typically UV radiation, to crosslink the exposed portions of the resin, followed by developing treatment in which the unexposed portions (non-crosslinked) of the resin are washed away with a developing liquid. Recent developments have introduced the CTP (computer-to-plate) method of creating the mask for the photosensitive resin. In this method, a thin (generally 1-5 μm in thickness) light absorption black layer is formed on the surface of the photosensitive resin plate and the resulting printing plate is irradiated imagewise with an infrared laser to ablate portions of the mask on the resin plate directly without separately preparing the mask. In such systems, only the mask is ablated without ablating the photosensitive plate precursor. Subsequently, the photosensitive plate precursor is imagewise exposed to UV light through the ablated areas of the mask, to crosslink (or harden) the exposed portions of the photosensitive resin, followed by developing treatment in which the unexposed portions (uncrosslinked) of the resin and the remaining black mask layer are washed away with a developing liquid. Both these methods involve a developing treatment that requires the use of large quantities of liquids and solvents that subsequently need to be disposed of. In addition, the efficiency in producing flexographic printing plates is limited by the additional drying time of the developed plates that is required to remove the developing liquid and dry the plate. Often additional steps of post-UV exposure or other treatments are needed to harden the surface of the imaged printing plate.
While the quality of articles printed using flexographic printing plates has improved significantly as the technology has matured, physical limitations related to the process of creating a relief image in a printing member still remain.
In the flexographic printing process, a flexographic printing member having a three-dimensional relief image formed in the printing surface is pressed against an inking unit (normally an Anilox roller) in order to provide ink on the topmost surface of the relief image. The inked raised areas are subsequently pressed against a suitable substrate that is mounted on an impression cylinder. As the flexographic printing member and Anilox or substrate are adjusted or limited mechanically, the height of the topmost surface determines the amount of physical impression pressure between the flexographic printing member and the Anilox or the flexographic printing member and the substrate. Areas in the relief image that are raised higher than others will produce more impression than those that are lower or even recessed. Therefore, the flexographic printing process is highly sensitive to the impression pressure that may affect the resulting image. Thus, the impression pressure must be carefully controlled. If the impression pressure is too high, some image areas can be squeezed, and if it is too low, ink transfer is insufficient. To provide the desired images, a pressman may test impression pressure settings for a given flexographic printing plate.
In particular, it is very difficult to print graphic images with fine dots, lines, and even text using flexographic printing members. In the lightest areas of the image (commonly referred to as “highlights”), the density of the image is represented by the total area of printed dots in a halftone screen representation of a continuous tone image. For Amplitude Modulated (AM) screening, this involves shrinking a plurality of halftone dots located on a fixed periodic grid to a very small size, the density of the highlight being represented by the area of the halftone dots. For Frequency Modulated (FM) screening, the size of the halftone dots is generally maintained at some fixed value, and the number of randomly or pseudo-randomly placed halftone dots represent the density of the image. In both of these situations, it is necessary to print very small dot sizes to adequately represent the highlight areas.
Maintaining small halftone dots on a flexographic printing member is very difficult due to the nature of the plate making process and the small size and lack of stability in the halftone dots. Digital flexographic printing precursors usually have an integral UV-opaque mask layer coated over a photopolymer or photosensitive layer in the relief image. In a pre-imaging (or post-imaging) step, the floor of the relief image in the printing member is set by area exposure to UV light from the back of the printing precursor. This exposure hardens the photopolymer to the relief depth required for optimal printing. This step is followed by selective ablation of the mask layer with an imagewise addressable high power laser to form an image mask that is opaque to ultraviolet (UV) light in non-ablated areas. Flood exposure to image-forming UV radiation and chemical processing are then carried out so that the areas not exposed to UV are removed in a processing apparatus using developing solvents, or by a heating and wicking process. The combination of the mask and UV exposure produces relief halftone dots that have a generally conical shape. The smallest of these halftone dots are prone to being removed during processing, which means no ink is transferred to these areas during printing (the halftone dot is not “held” or formed on the printing plate or on the printing press). Alternatively, if the small halftone dots survive processing, they are susceptible to damage on press. For example, small halftone dots often fold over or partially break off during printing, causing either excess ink or no ink to be transferred.
Conventional preparation of non-digital flexographic printing plates follows a similar process except that the integral mask is replaced by a separate film mask or “phototool” that is imaged separately and placed in contact with the flexographic printing precursor under a vacuum frame for the image-forming UV exposure.
A solution to overcome the highlight problem noted above is to establish a minimum halftone dot size during printing. This minimum halftone dot size must be large enough to survive processing and be able to withstand printing pressure. Once this ideal halftone dot size is determined, a “bump” curve can be created that increases the size of the lower halftone dot values to the minimum halftone dot setting. However, this results in a loss of the dynamic range and detail in the highlight and shadow areas. Overall, there is less tonality and detail in the image.
Thus, it is well known that there is a limit to the minimum size of halftone dots that can be reliably represented on a flexographic printing member and subsequently printed onto a receiver element. The actual minimum size will vary with a variety of factors including printing flexographic printing member type, ink used for printing, and imaging device characteristics among other factors including the particular printing press that is used. This creates a problem in the highlight areas when using conventional AM screening since once the minimum halftone dot size is reached, further size reductions will generally have unpredictable results. If, for example, the minimum size halftone dot that can be printed is a 50×50 μm square dot, corresponding to a 5% tone at 114 lines per inch screen frequency, then it becomes very difficult to faithfully reproduce tones between 0% and 5%. A common design around this problem is to increase the highlight values in the original file to ensure that after imaging and processing, all the tonal values in the file are reproduced as printing dots and are properly formed on the printing member. However, a disadvantage of this practice is the resulting additional dot gain in the highlights that causes a noticeable transition between inked and non-inked areas.
Another known practical way of improving highlights is through the use of “Respi” or “double dot” screening. One such technique is illustrated in FIGS. 1-A to 1-C. Screening grid 10 for a conventional AM screen is shown in a simplified schematic format and comprises a plurality of halftone cells 12. A halftone cell is an area wherein a printed AM halftone dot is grown from a low density, where only a small dot is placed at the center of the cell, to a high density or solid where the cell is completely filled. In FIG. 1-A, dots 14 are placed only in every second halftone cell and have a size corresponding to the minimum reliably reproducible dot. As the density of the screen is increased, the dots 16 are increased in size as shown in FIG. 1-B. At some point in increasing the density, the previously empty halftone cells are populated with minimum size dots 18 as shown in FIG. 1-C. Dots 16 may be held at a fixed size with increasing screen density while allowing dots 18 to grow. When all dots are the same size as dots 16, conventional AM screening takes over.
The problem with this type of screening technique, when applied to flexographic printing, is that the size of halftone dot that may be printed in isolation is actually quite large, typically 40-50 μm in diameter. Even when using this technique, the highlights are difficult to reproduce without having a grainy appearance, which occurs when halftone dots are spaced far apart to represent a very low density, and the printed halftone dot may also suffer an undesirable dot gain.
U.S. Pat. No. 7,486,420 (McCrea et al.) discloses a flexographic screening technique that compensates for characteristic printing problems in highlight areas by selectively placing non-printing dots or pixels proximate to highlight dots. The non-printing dots or pixels raise the printing relief floor in the highlight areas providing additional support for marginally printable image features. This technique allows an image feature to be surrounded by one or more smaller non-printing features to provide an extra base of support for the image feature. While this provides an important advance in the art, it may not always completely eliminate the grainy appearance in the image.
MAXTONE screening (Eastman Kodak Company) is a known hybrid AM screening solution that overcomes some highlight and shadow reproduction limitations. MAXTONE screening software allows the operator to set a minimum dot size in order to prevent the formation of halftone dots that are too small for the flexographic medium. To extend the tonal range, MAXTONE screening software uses an FM-like screening technique in the highlights and shadows. To create lighter shades, dots are removed in a random pattern. By producing lighter colors with fewer (rather than smaller) halftone dots, improved highlight detail and a more robust flexographic printing plate are achieved. However, completely removing dots from a highlight will necessarily reduce the resolution and edge fidelity of the resulting printed images.
U.S. Pat. Nos. 5,892,588 and 6,445,465 (both Samworth) describe an apparatus and method for producing a halftone screen having a plurality of halftone dots arrayed along a desired screen frequency by deleting a number of halftone dots per unit area to obtain gray shades below a predetermined shade of gray.
A Feb. 1, 2010 publication by the Association of Japanese Flexo Printing Industry entitled “Direct Laser Plate Making Consideration for Current Status” describes the use of undercutting in preparing flexographic printing plates to release the printing pressure in the highlight areas. FIG. 7 in this publication shows a progressive and predetermined undercutting in the relief image.
It is known to undercut certain dots as shown in FIGS. 2a to 2c wherein line 30 represents the top surface of the flexographic printing member coincident with the top surface of a solid or border 100. Dots 50 represent highlight dots in highlight areas. Line 20 represents the floor of the relief image, or the lowest plane of the engraved printing member and the depth between 30 and 20 is called the relief. Base 60 supports the floor and can be composed of various materials or composites including a rubber, woven cloth, or a polymer support. In FIG. 2a, none of highlight dots 50 is undercut and all of highlight dots 50 are printable. In FIGS. 2a-2c, the walls of the dots (for example, dots 50) are schematically represented as vertical. However, it is known that the actual dots can have sloped walls or walls with varying slopes or combinations of slopes and plateau regions between the top surface of the halftone dot and the floor. FIG. 2b shows an example of a prior art printing surface wherein all highlight dots 50 are equally undercut by amount 40. This commonly occurs due to exposure and process conditions in digital UV processed flexographic plates and extreme undercutting is undesirable because the undercut depth will exceed the lowest plane 49 that will transfer ink on press and will have no ink density on the printed receiver. FIG. 2c shows an example of the prior art where non-printing dots 70, having top surfaces below lowest plane 49 are intentionally added to raise the printing relief floor above line 20 in the highlight areas, providing additional support for marginally printable image features. This technique, however, provides only marginal improvement in highlight fidelity.
U.S. Pat. No. 6,984,478 (Dreber et al.) describes print control for flexographic printing by using a print control element.
Despite all of the progress made in flexographic printing to improve image quality in the highlight areas, there remains a need to improve the representation of small halftone dots in printed flexographic images so that image detail is improved and dot gain is reduced.